Abstract:
Have been studied 13 andosol samples from two profiles (Gurghiu
Mountains), from chemical and mineralogical point of view. The field surveys
and chemical-mineralogical analysis have evidenced that the andic character
of studied soils, the andesitic nature of parental material by eruptive type and
a different spatial distribution of andosols towards to the underlined in
previous studies. The andosols from Gurghiu Mountains have been formed
on andesitic material arise from two sources, in two successive steps: (i) in
first step on basis of piroclastes materials (ash and volcanic glass), (ii) in
second step on basis of andesitic agglomerates. The frequent occurrence
form of piroclastes in studied andosols (> 90 %) is the granules associated
with other mineral and organic components: (i) macroscopic associations
(agglomerates) and (ii) colloidal microscopic associations (macromicelles).
Over 80 % from amorphous clay minerals (imogolite, allophane) and from
humus are included in allophano-humic associations. The structure and the
ratio between the components of allophano-humic complexes varied in
function of allophonic or non-allophanic character of andosols. Another
characteristic of these andosols is the existence of two types of allophanes
and the specific way of association of these with humus and other mineral
components of andosols: (i) allophone I (aluminous) characteristic to the
non-allophanic andosols and (ii) allophone II (siliceous) characteristic to the
allophanic soils. Between these two forms of allophane is a dynamic
equilibrium, conditioned by pH, organic matter content and thermodynamic
activity of Al, Si and Fe. The transformation of allophane I in allophane II is
accompanied by the retention of organic matter in allophanic-humic
complexes, this having a protector role for allophane. At pH values lower
than 5, the organic matter manifest and an anti-allophanic effect, which is
traduced by destabilization of allophanic-humic associations, with the
formation of metal (Al, Si) – humus complexes. In case of studied andosols
the anti-allophanic effect is relative intense and has a regular character,
dependent by the physic-chemical conditions from soil.
